A long time back, Black Mesa was released. This is a Source mod that is a recreation of Half-Life. It has been in development since 2004 or so, when Half-Life: Source was released, and everyone discovered that it was pretty much the same thing from 1998. Originally this mod was called "Black Mesa: Source", but Valve kindly asked if they would drop the "Source" part.
The whole thing is a semi-verbatim recreation of Half-Life. Some parts have been eliminated, others shortened, and a few levels have been expanded. Oh, and it looks prettier. Much prettier, in fact. There is nothing here that looks like poop smeared origami. I recall the building with the lasers and the nuke was changed quite a bit, because the minefield getting inside there didn't make me angry. The area where a grenade explosion happens in a tube and you have to back out of it has been eliminated. The "on a rail" level has been shortened quite a bit; which is too bad, because that was one of my favorite levels. I heard about a mod to this that expands those areas, but I haven't played it.
All of the assets are original, mostly because this is not Valve's game, and the 14+ year gap between it and it's... inspiration. The music is fantastic, and the guy was nice enough to put it up for sale. I remember playing Oblivion back in the day, and I took some pre-released music from Black Mesa and put it into the music folder. It was good back then, and it seemed quite appropriate. Of course, that just goes to show me how long Black Mesa has been in production.
Speaking of being in production, it still is, as the final alien levels were not released back in September. I guess they wanted everyone to know that there was actually stuff going on, and it was worth waiting some more for. And you better enjoy it, because that's all the Half-Life there ever will be, since it is a dead series. There has been more time since the last installment than between the original and its sequel. Since then, there has been another popular game series about an alien invasion, that, well... you know.